Structure and intermolecular hydrogen bond of jet-cooled p-aminophenol-(H₂O)₁ studied by electronic and IR-dip spectroscopy and density functional theory calculations

The structure and hydrogen bonding interaction in jet-cooled p-aminophenol-H₂O 1:1 complex have been studied by measuring the fluorescence excitation, dispersed fluorescence, and IR-dip spectra. In the electronic spectrum we identified only one isomer, where the oxygen atom of water is bonded to the hydroxy proton of p-aminophenol. Four stable isomers are obtained by ab initio calculations at the MP2/6-31G(d) level, while density functional theory calculations provide four or three isomers depending on the basis sets. It has been shown that theoretical IR spectra with small basis sets are not in agreement with the experimental IR spectrum. The experimental IR spectrum has been well reproduced by the B3LYP/6-311+G(d,p) calculations, showing that diffuse functions are necessary to describe the intermolecular hydrogen bond in p-aminophenol-H₂O. The vibronic levels in the S₁ state of p-aminophenol-H₂O have been assigned with the aid of the dispersed fluorescence spectra. The formation of the intermolecular hydrogen bond substantially reduces the frequency of the amino inversion mode in the S₁ state due to nonlocal character of this mode.